Optically variable devices are used in a wide variety of applications, both decorative and utilitarian. Optically variable devices can be made in variety of ways to achieve a variety of effects. Examples of optically variable devices include the holograms imprinted on credit cards and authentic software documentation, color-shifting images printed on banknotes, and enhancing the surface appearance of items such as motorcycle helmets and wheel covers.
Optically variable devices can be made as film or foil that is pressed, stamped, glued, or otherwise attached to an object, and can also be made using optically variable pigments. One type of optically variable pigment is commonly called a color-shifting pigment because the apparent color of images appropriately printed with such pigments changes as the angle of view and/or illumination is tilted. A common example is the “20” printed with color-shifting pigment in the lower right-hand corner of a U.S. twenty-dollar bill, which serves as an anti-counterfeiting device.
Some anti-counterfeiting devices are covert, while others are intended to be noticed. Unfortunately, some optically variable devices that are intended to be noticed are not widely known because the optically variable aspect of the device is not sufficiently dramatic. For example, the color shift of an image printed with color-shifting pigment might not be noticed under uniform fluorescent ceiling lights, but more noticeable in direct sunlight or under single-point illumination. This can make it easier for a counterfeiter to pass counterfeit notes without the optically variable feature because the recipient might not be aware of the optically variable feature, or because the counterfeit note might look substantially similar to the authentic note under certain conditions.
Optically variable devices can also be made with magnetic pigments that are aligned with a magnetic field after applying the pigment (typically in a carrier such as an ink vehicle or a paint vehicle) to a surface. However, painting with magnetic pigments has been used mostly for decorative purposes. For example, use of magnetic pigments has been described to produce painted cover wheels having a decorative feature that appears as a three-dimensional shape. A pattern was formed on the painted product by applying a magnetic field to the product while the paint medium still was in a liquid state. The paint medium had dispersed magnetic non-spherical particles, commonly referred to as flakes, that aligned along the magnetic field lines. The field had two regions. The first region contained lines of a magnetic force that were oriented parallel to the surface and arranged in a shape of a desired pattern. The second region contained lines that were non-parallel to the surface of the painted product and arranged around the pattern. To form the pattern, permanent magnets or electromagnets with the shape corresponding to the shape of desired pattern were located underneath the painted product to orient in the magnetic field non-spherical magnetic particles dispersed in the paint while the paint was still wet. When the paint dried, the pattern was visible on the surface of the painted product as the light rays incident on the paint layer were influenced differently by the oriented magnetic particles.
Similarly, a process for producing of a pattern of flaked magnetic particles in fluoropolymer matrix has been described. After coating a product with a composition in liquid form, a magnet with desirable shape was placed on the underside of the substrate. Magnetic flakes dispersed in a liquid organic medium orient themselves parallel to the magnetic field lines, tilting from the original planar orientation. This tilt varied from perpendicular to the surface of a substrate to the original orientation, which included flakes essentially parallel to the surface of the product. The planar oriented flakes reflected incident light back to the viewer, while the reoriented flakes did not, providing the appearance of a three dimensional pattern in the coating.
While these approaches describe methods and apparatus for formation of three-dimensional-like images in paint layers, they are not suitable for high-speed printing processes because they are essentially batch processes. U.S. Pat. No. 3,873,975 to Miklos et al issued Mar. 25, 1975, discloses a magnetic system for authentication and interrogation of security documents such as credit cards, airline tickets etc, which utilizes magnetic recording of security patterns in a record medium having magnetically detectable permanent fixed information pattern. Such a record medium is prepared by providing a sheet having a non-magnetic backing and a layer thereon incorporating magnetically anisotropic magnetizable particles which are temporarily free to rotate. The magnetizable particles at selected locations in the layer are magnetically aligned to form a fixed information pattern, such as an alphameric character, by passing the sheet adjacent a rotatable cylinder. The cylinder has a plurality of small permanent magnets mounted on a non-magnetic outer surface of the cylinder in a pattern corresponding to the fixed information pattern and has the magnets oriented to provide substantially unidirectional magnetic fields in the layer when adjacent thereto. The cylinder is rotated as the sheet is passed adjacent thereto to apply the magnetic fields in the layer to physically align the magnetizable particles at the selected locations to implant the fixed information pattern in the layer. The imprinted patterns form a security feature that can be magnetically read for document authentication.
The apparatus disclosed by Miklos et al, although apparently enabling a continuous magnetic printing of pre-determined two-dimensional security patterns by orienting magnetic particles in a recording media, has certain disadvantages.
First, the device of Miklos et al can magnetically imprint only substantially two-dimensional patterns copying geometrical arrangements of the magnets mounted on the cylinder, each magnet essentially providing a “dot” in the pattern imprinted in the magnetic layer. It would be very difficult if at all possible to use this technology to provide pre-determined substantially three-dimensional (3D) or complex arrangements of magnetic flakes that are required for providing optical images with variable or illusive optical effects, such as rolling objects and images with illusion of depth.
Second, in the device of Miklos et al, the magnets are mounted on the outer surface of the cylinder, and are therefore projecting therefrom. This can be highly disadvantageous in contact printing, when the cylinder is in contact, possibly under some pressure, with the sheet whereupon the magnetic layer is disposed.
Third, since in the device of Miklos et al, the magnets are fixedly attached to the surface of the cylinder, it cannot be used to form illusive images that can be produced by spinning magnets, as described in co-pending U.S. patent application Ser. No. 11/278,600, filed Apr. 4, 2006, the disclosure of which is incorporated herein by reference.
European patent application EP 1493590 describes a method and means for producing a magnetically induced design in a coating containing magnetic particles that also uses a rotatable cylinder to orient magnetic particles in pre-determined patterns. The device comprises a body of a composite permanent magnetic material having at least one flat or curved surface engraved with the pattern corresponding to the pattern of desirable indicia. The magnetic material is magnetized in the direction perpendicular to the surface. Irregularities in the surface, made with an engraving, produce changes in the direction and strength of the resulting magnetic field. These changes cause different alignment of magnetic particles in different parts of the wet ink that make possible a formation of an image with a shape corresponding to the shape of engraving. The device can be a permanently magnetized flexible plate mounted on a rotatable cylinder of a printing press, with the engraved surface having surface irregularities in the shape of the indicia located at the outer surface of the cylinder. The approach of EP 1493590 has however certain limitations. The steps required to produce the engravings in the permanent magnetic material can be cumbersome; furthermore, the printing with the engraved surfaces is generally directed to reproducing the engraved indicia or drawings, and is limited in terms of optical effects it can produce. For example, the inventors of this invention have found that, when printing in accordance with the teachings of EP 1 493 590 using color-shifting inks, a very poor color-shifting effect resulted. A color-shifting affect has the best appearance, i.e. large color travel and high chroma value, when the particles providing the effect are parallel or almost parallel to the surface of the substrate, which is hard to achieve using the device of EP 1 493 590.
It is therefore desirable to provide an apparatus for a high-speed in-line printing and painting that re-orients magnetic pigment flakes in pre-determined substantially 3D patterns for providing illusive and/or variable optical effects, and which would not involve using printing surfaces with protruding magnets.
It is also desirable to provide a rotatable roller for a high-speed in-line printing that can be easily assembled and disassembled for forming various combinations of optical images and variable or illusive optical effects.
It is another object of the present invention to provide an apparatus for continuous in-line printing that uses spinning magnets to orient magnetic flakes in pre-determined substantially 3D patterns for providing optical images with the illusion of depth.